There are numerous potential applications of carbon nanotubes (CNTs) because of their unique mechanical, physical, electrical, chemical, and biological properties. For example, ultra low resistance conductors, semiconductors, highly efficient electron emitters, ultra-strong lightweight fibers for structural applications, lasers, and gas sensors can all be manufactured by using CNTs. For reviews of CNT technology, properties, and applications, see Baughman et al., “Carbon Nanotubes—the Route Toward Applications”, Science, volume 297, pages 787-792 (2002); Michael J. O'Connell (Editor) “Carbon Nanotubes—Properties and Applications”, CRC Taylor & Francis, New York (2006); Yury Gogotsi (Editor) “Nanomaterials Handbook”, CRC Taylor & Francis, New York (2006).
The incorporation of non-carbon materials into CNTs may lead to even more diverse range of applications, for example, in improved gaseous storage media or electronic devices. In a publication entitled “Titanium-Decorated Carbon Nanotubes as a Potential High-Capacity Hydrogen Storage Medium”, Physical Review Letters, 2005, Vol. 94, article 175501, Yildirim et al. described that each titanium atom adsorbed on a single-wall CNT (SWCNT) may theoretically bind up to four hydrogen molecules. Thus, high-capacity hydrogen storage equipment may be prepared from such materials, if they were available.
A variety of synthesis techniques for preparing CNTs exist. These techniques include for example carbon arc, laser ablation, chemical vapor deposition, high pressure carbon monoxide process (HiPco), cobalt-molybdenum catalyst process (CoMoCat). Depending on the preparation method, CNTs may be metallic and semiconducting. To improve electrical conductivity of semiconducting CNTs, non-carbon materials such as metals can be incorporated into CNTs for their conversion into conducting materials.
In a publication entitled “Titanium Monomers and Wires Adsorbed on Carbon Nanotubes: A First Principles Study,” Nanotechnology, 2006, Vol. 17, pages 1154-1159, Fagan et al. described that a metallic Ti-wire/tube system may potentially be obtained by incorporating titanium in a semiconductor SWCNT. As a result, the electrical conductivity of such materials may reach to a level comparable or even surpassing that of copper. Such materials, if available, may aid in the advance of electronic applications. Fagan et al. also described that the Ti monomer or wire adsorbed into a SWCNT could be more stable than that adsorbed on outside surface of the SWCNT.
Gao et al. in U.S. Pat. Nos. 6,361,861 and 7,011,771 hypothesized the formation of titanium carbide, silicon carbide, and tantalum carbide core in carbon nanotubes. They disclosed a method by which TiC filled CNTs were grown on a growth catalyst and a titanium substrate. Energy Dispersive X-Ray (EDX) analysis of the nanorods thereby prepared revealed that the cores are cubic TiC.
In a publication entitled “Synthesis and Characterization of Carbide Nanorods,” Nature, 1995, vol. 375, pages 769-772, Dai et al. described that when TiO or Ti+I2 were reacted with carbon nanotubes, TiC nanorods were obtained. These nanorods were analyzed by X-Ray Diffraction (XRD) and found no evidence for presence of graphitic (nanotube), Ti-metal or Ti-oxide peaks.
Guerret-Plecourt et al. in a publication entitled “Relation between Metal Electronic Structure and Morphology of Metal Compounds Inside Carbon Nanotubes” Nature, 1994, vol. 372, pages 761-765, the arc-discharge method also yielded only TiC filled CNTs.
Nagy et al. in a European Patent No. 1 401 763 B1 disclosed preparation of carbon nanotubes on Ti(OH)4 supported Fe—Co catalysts. The MWCNTs thereby prepared were later purified and then analyzed by Proton Induced Gamma Ray Emission and Proton Induced X-ray Emission. Nagy et al. found no evidence for incorporation of Ti in the carbon nanotubes. Thus, previous attempts to fill CNTs with titanium compounds either failed or resulted in formation of TiC.
Formation of metal carbides during incorporation of non-carbon materials may alter the structure of CNTs, resulting in articles with poor electronic, thermal, chemical and mechanical properties or articles with properties different than those targeted. Therefore, this formation should be limited in order to obtain articles with desired properties.
With respect to carbon nanotube separation, the as-synthesized SWCNTs using the existing techniques are not pure. They may comprise amorphous carbon and metal catalysts. The as-synthesized SWCNTs may further comprise metallic and semiconducting carbon nanotubes. Semiconducting SWCNTs are hereafter abbreviated as s-SWCNTs and metallic SWCNTs as m-SWCNTs. The as-synthesized SWCNTs may also comprise carbon nanotubes with a variety of diameters. The relative amount of each component present in as-synthesized SWCNTs depends on the synthesis process used.
To utilize their unique properties, the as-synthesized SWCNTs should be separated into their components. For example, it may be required to remove the amorphous carbon and the catalyst from the as-synthesized SWCNTs to utilize their electric properties. Further separation according to their electrical conductivities, which depend on their chiralities and diameters, may also be required if the application is of semiconducting or conducting type. For example, the use of SWCNTs as transistor channels requires s-SWCNTs and the use of SWCNTs as conductors for on-chip connects requires m-SWCNTs. Furthermore, the semiconductor properties also depend on the diameter of SWCNTs. Their semiconductor band-gaps decrease with increasing diameter. If they comprise s-SWCNTs with varying diameters, they will have a band-gap varying in a wide range. Thus, separation of SWCNTs according to their diameters may yield s-SWCNTs with semiconductor band-gaps varying in a narrower and better defined range, making them suitable for electronic devices based on Schottky barriers.
In sum, there exists a need for new or improved carbon nanotube materials with greater purity and superior performance and methods of making these materials. Also needed are practical separation procedures to prepare SWCNTs containing size-specific and/or chiral-specific populations. A size or chirality enriched population of SWCNTs is useful in the preparation and manufacture of commercial products or components thereof. They can also be used as intermediates for preparing other desired products.